Boiling Water Reactor (BWR) fuel assemblies include fuel rods and water rods within a flow channel. Water flowing through the many fuel bundles within respective channels provide both coolant and moderator to sustain the nuclear reaction. The moderator function is provided primarily by the higher density liquid. Energy addition along the fuel rods, however, converts some of the water to lower density steam so that its effectiveness as a moderator decreases as the fraction of liquid decreases along the length of the fuel rods. The resultant steam-water flows, referred to as two-phase flows, have higher velocities and cause significant pressure drop along the length of the fuel during typical operation. To reduce this variation in liquid moderator, modem BWR fuel designs include separate flow paths within the fuel bundles which remain filled with liquid (water) over the length of the fuel bundles. These water paths can be configured as one or more round or square tubes (or as cruciform shaped flow passages) generally referred to as water rods. For normal operating conditions, these water rods provide 15% to 20% of the available moderator liquid in BWR fuel bundles.
Current water rod designs provide parallel flow paths through the fuel bundle. They typically include one or more inlet holes near the bottom of the water rods (and the fuel bundle) and one or more outlet holes near the top of the water rods (and the fuel bundle), as illustrated in FIGS. 1 and 2. The placement of these inlet and outlet holes imposes the full bundle pressure drop to drive flow through the water rods. When reactor flow reduces, the pressure difference driving liquid through water rods is also reduced. Current water rods are designed with appropriate inlet flow resistances to maintain very little vapor formation even for such low flow conditions.
A characteristic of such two-phase flow systems is that they can experience flow oscillations under certain conditions of low flow and high power operation. A BWR has a natural tendency to avoid such flow oscillations because the increased steam formed at low flow conditions causes the reactor power to reduce. However, since current water rods are typically designed to remain full of liquid even at low reactor flow conditions, those water rods limit the natural power reduction at low flow conditions. On the other hand, if the flow restrictions in the water rods are increased to produce significant steam in the water rods at low flow conditions, the current designs result in unfavorable steam in the water rods at normal operation conditions as well.
Two recent water rod designs have been proposed to improve the prior designs mentioned above. One recent design provides varying amounts of steam in the water rod at different reactor flow conditions. This design is intended primarily to enhance spectral shift capability. It utilizes a very small (in cross section) downflow extension tube from the top of the water rod. Thus, water flows upward through a large path and then downward through a small connecting tube for nearly the full length of the fuel before reaching the exit hole. This configuration (illustrated in FIG. 3) has only a short vertical distance between the inlet and outlet holes, with resultant small imposed pressure differential across these holes. For low flow conditions, the downward flow tube is predominantly filled with steam, and the fluid in the upward path is supported like a standpipe with a low pressure differential. The resultant liquid content in the tube is thus quite low, being proportional to the imposed pressure differential. For normal operation, however, the small downflow tube and significant outlet flow restriction combine to severely limit water rod flow. Thus, this design results in significant steam formation in the water rod, with associated unfavorable fuel efficiency, under normal operating conditions.
Another recent water rod design introduces a central standpipe for upflow within a normal water rod design (illustrated in FIG. 4). Using flow restrictions typical of current designs allows for sufficient water rod flow at normal operating conditions to avoid steam formation. For low flow conditions, it was contemplated that the annular region outside of the standpipe would fill with steam when the imposed pressure differential dropped below that necessary to spill liquid over from the top of the standpipe. Unfortunately, analyses have indicated that under such conditions, liquid will flow backward through the upper outlet hole and refill that annular region outside the standpipe (since that region has no bottom drain, it can potentially collect even more liquid than current water rod designs under similar conditions). The only known way to avoid such liquid backflow at the upper outlet hole is to introduce sufficient restriction such that backflow is avoided by the counter flow effects of the escaping vapor. Unfortunately, that amount of flow restriction will again cause unfavorable steam formation in the water rod at normal operating conditions.